Over the course of the first year, typically-developing infants rapidly acquire skills in object-directed reaching and postural control. These skills underlie increasingly sophisticated object exploration behaviors that yield information not only about objects in the world but about the effects of the infant's own actions on those objects. Object exploration and the knowledge that it generates is, in turn, foundational for the later development of language and communication. This suggests that the emergence of reaching and the progressive achievement of greater postural control, mediated by object exploration, may have cascading and far-reaching effects on later development, not only on more advanced motor planning skills, but in domains such as communication and language that are not typically conceived as being influenced by early motor development. Conversely, delays and impairments in reaching and postural development may negatively impact object exploration and, therefore, the eventual development of later, higher-order developments in motor planning, language, and communication. The work proposed in this application is designed to shed light on these previously unexplored developmental pathways by providing systematic longitudinal data on the emergence of reaching, the growth of postural control, changes in object exploration behaviors, and later developments in motor planning, language, and communication from 2.5 to 36 months of age. The research is innovative because it employs a dense, longitudinal home-visit observation schedule and combines behavioral and kinematic data to permit close tracking of advances in reaching, self-sitting and object exploration. This multi-method approach will support a detailed examination of normative and atypical developmental trajectories in these domains and their relationship to the development of later-emerging behaviors both within and across domains of development. By focusing not only on typically-developing, low-risk (LR) infants but also on infants known to be at heightened risk (HR) for communicative delays (viz., infants with an older brother or sister diagnosed with Autistic Disorder), we will be able to assess early patterns of delay and examine any negative cascading effects that they may have This, in turn, will lay the groundwork for improvements in clinical practice and design of interventions to remediate early delays and avoid their cascading effects. Successful completion of this project will result in: a) clarification of the nature of the relationship between the growh of postural control, the development of infant reaching, and improvement in object exploration skills during the first year; b) specification of the nature and extent of delays and impairments i postural control, reaching, and object exploration in HR infants; and c) identification of any predictive relations existing between early postural and motor development and later, higher-order motor, language, and communication skills as mediated by the growth of object exploration.